We propose to search the monophoton events at the BESIII detector and future Super Tau Charm Factory to probe the sub-GeV dark photon decay into lighter dark matter. We compute the cross section due to the dark photon associated a standard model photon production, and study the corresponding standard model irreducible/reducible backgrounds. By using the data about 17 fb −1 collected at the BESIII detector since 2011, we derive new leading limits of the mixing strength ε, ε (1.1 − 1.6) × 10 −4 , in the mass range of 0.04 GeV m A 3 GeV. With 30 ab −1 data, STCF running at √ s = 2 GeV, can probe ε down to 5.1×10 −6 when m A = 1 GeV. For models of scalar and fermionic light thermal dark matter production via dark photon, we present the constrains on the dimensionless dark matter parameter y = ε 2 αD(mχ/m A ) 4 as function of the DM mass mχ at BESIII and future STCF, conventionally assuming the dark coupling constant αD = 0.5 and m A = 3mχ. We find that BESIII can exclude model of scalar, Majorana, and pseudo-Dirac (with a small splitting) DM for the mass region 0.03∼1 GeV, 0.04∼1 GeV and 0.4∼1 GeV respectively. For values αD 0.005, combining the results from 2 GeV STCF with 30 ab −1 data and BaBar, one can exclude the above three DM models in the mass region 0.001 GeV mχ 1 GeV.